You strip the girl by breaking special hidden bumpers. They normally need 3 to 5 hits to break. If you hold the ball still for a while you can watch them light up now and then to hint where they are located on the board.
If you meant to ask about how to unlock sexplay section; that is done by filling the heart meter to the top - which can only happen while at least the top or bottom is naked. Alternatively, you can unlock the sexplay section in the shop with coins.